1. Field of the Invention
The present invention relates to a pickup for a magneto-optical recording medium, for recording information into magneto-optical recording media and/or for reproducing information from the magneto-optical recording medium.
2. Description of the Related Art
A widely known example of a recording medium for recording information by using a magneto-optical recording system may be a mini disc (abbreviated as MD) having a diameter of 64 mm for recording music information. In addition to the MD, the recording medium employing the magneto-optical recording system can be represented by an MO disk used for an MO (magneto-optical) disk drive which is an external storage unit for a computer. There have now been used MO disks of various standards having different recording systems and different recording film structures.
There have been proposed various ultra-resolution technologies for strikingly improving the recording capacities of the magneto-optical recording/reproducing systems that deal with the MDs or the MO disks. For example, there is a method called magnetic field modulation recording system for recording marks smaller than a diameter of a light beam. As methods of reproducing marks smaller than the diameter of a light beam, further, there have been proposed a magnetic super-resolution technology called MSR (Magnetic SuperResolution), a magnetic domain expanding reproduction method called MAMMOS (Magnetic AMplifying Magneto Optical System) and a magnetic wall displacement detection technology called DWDD (DomainWall Displacement Detection). The magneto-optical recording medium (disk) to which the DWDD system is applied is constituted by at least three magnetic film layers, i.e., a domain wall displacement layer, a switching layer and a magnetic recording layer (memory layer), and in which the size of a magnetic domain effectively recorded is expanded in a region where the temperature of the magnetic film has exceeded the Curie temperature of the switching layer by utilizing the displacement of the magnetic wall in the magnetic wall displacement layer in the case of information reproduction thereby to increase the reproduced carrier signals.
There have further been proposed optical pickups based on the magneto-optical recording system in various constitutions concerning the optical system one of the related arts that are proposed, such as Japanese Unexamined Patent Publication JP-A 8-297875 (1996), may be an integrated unit in which a detection system for detecting servo signals and magneto-optical signals is incorporated in a package and an optical pickup of a decreased size. FIG. 18 is a side view of an arrangement illustrating, in a simplified manner, the constitution of an optical pickup 1 according to a related art, and FIG. 19 is a plan view of an arrangement of a light detector 7 provided in the optical pickup 1 illustrated in FIG. 18.
The optical pickup 1 according to the related art includes a semiconductor laser element 2, a grating 3, first polarization/separation means 4, an objective lens 5, a second polarization/separation means 6, and a light detector 7. The semiconductor laser element 2, grating 3, first polarization/separation means 4, second polarization/separation means 6 and light detector 7 are incorporated in a package 8. The first polarization/separation means 4 is formed on a surface of a glass substrate 9 which is a base material facing the objective lens 5, and the second polarization/separation means 6 is formed on a surface of the glass substrate 9 on the side opposite to the surface on where the first polarization/separation means 4 is formed, i.e., on the surface facing the semiconductor laser 2. The first polarization/separation means 4 is a polarizing hologram having an enhancing function formed on, for example, a birefringent substrate, and has an optical axis that is so set as to be perpendicular to the direction of polarization of the light beam from the semiconductor laser element 2, permitting ordinary light to pass through and diffracting extraordinary light.
The light beam going out from the semiconductor laser element 2 passes through the first polarization/separation means 4 and is projected onto an information recording surface of an MO disk 10 (abbreviated as MOD) which is a magneto-optical recording medium. A light beam (hereinafter called return light beam) reflected by the MOD 10 passes through the objective lens 5 again and falls on the first polarization/separation means 4. The return light beam falling on the first polarization/separation means 4 is diffracted into at least zero (0)-order diffracted light and plus and minus (±) first-order diffracted lights through the first polarization/separation means 4. The ± first-order diffracted lights of the return light beam diffracted by the first polarization/separation means 4 fall on the second polarization/separation means 6.
The second polarization/separation means 6 comprises members split into two, i.e., second polarization/separation means (I) 6a and second polarization/separation means (II) 6b, which are arranged being divided into the right and the left on the surface of the paper in FIG. 18. The second polarization/separation means (I) 6a and the second polarization/separation means (II) 6b are polarizing holograms which are so constituted as to diffract extraordinary light. The second polarization/separation means 6 is so arranged that the optical axis of the substrate is at an angle of 45 degrees with respect to the direction of polarizing the light beam from the semiconductor laser element 2, and permits ordinary light to pass through while diffracting extraordinary light.
The + first-order diffracted light diffracted through the first polarization/separation means 4 falls on the second polarization/separation means (I) 6a which permits the zero-order diffracted light that is incident to pass through while refracting the − first-order diffracted light. The − first-order diffracted light diffracted through the first polarization/separation means 4 falls on the second polarization/separation means (II) 6b which permits the zero-order diffracted light that is incident to pass through while refracting the + first-order diffracted light. The zero-order light and the ± first-order diffracted light diffracted through the second polarization/separation means (I) 6a and (II) 6b fall on the portions 7a and 7b of the light detector 7. Namely, the zero-order diffracted light and the − first-order diffracted light through the second polarization/separation means (I) 6a fall on the portion 7a of the light detector 7, and the zero-order diffracted light and the + first-order diffracted light through the second polarization/separation means (II) 6b fall on the portion 7b of the light detector 7.
Referring to FIG. 19, the light detector 7 includes a first light detector 7a and a second light detector 7b separated into the right and the left with respect to the semiconductor laser element 2. The first and second light detectors 7a and 7b include main light-receiving portions 15, 12 for receiving main beams, and sub-light-receiving portions 14, 16 and 11, 13 for receiving sub-beams. The main light-receiving portions 12 and 15 are further divided into the right and the left. If the right and left outputs of the main light-receiving portion 12 are denoted by a and b, and the right and left outputs of the main light-receiving portion 15 by c and d, then, the read signal MO of the MOD 10 is given by the following formula (1). The method of detecting servo signals is not described here. According to the related art, the optical pickup is realized in a small size relying upon the constitution.MO=(a+d)−(b+c)   (1)
In the optical pickup 1 according to the related art, the polarizing hologram is used as the first polarization/separation means 4 for diffracting the return light beam from the MOD 10 toward the light detector 7. Here, however, the polarizing hologram includes an enhancing function for improving the quality of signals by increasing the degree of modulation of the magneto-optical reproduced signals. That is, the optical axis of the birefringent substrate constituting the polarizing hologram is set to be perpendicular to the direction of polarizing the light beam emitted from the semiconductor laser element 2 so that, for example, the diffraction efficiencies for the ordinary light (p-polarization) are 67% of zero-order light and 27% of t first-order light, and the diffraction efficiencies for the extraordinary light (s-polarization) are 18% of zero-order light and 76% of ± first-order light.
However, a phase difference is often included between the p-polarization and the s-polarization of the ± first-order diffracted lights for detecting the magneto-optical reproduced signals arousing a problem in that the quality of the reproduced signals is greatly deteriorated as the phase difference increases, i.e., as the linearly polarized light becomes the elliptically polarized light. As the magneto-optical recording media, further, there have been proposed ultra-resolution media having various recording film structures. In the DWDD system of the magnetic multi-layer film structure, however, even when the linearly polarized light is incident on the magneto-optical recording medium, there takes place a case where the return light beam itself modulated by Kerr rotation angle returns back as the elliptically polarized light, i.e., there takes place a case where a phase difference occurs between the polarized component of the incident light and a component perpendicular thereto. In this case, too, the quality of the reproduced signals is greatly deteriorated like when a phase difference occurs between the p-polarization and the s-polarization of the ± first-order diffracted lights through the polarizing hologram. Therefore, there remains a problem in that a single optical pickup cannot cope with the magneto-optical recording media of different recording/reproducing systems.